Microscopic phytoplankton, green plants that use carbon dioxide producing food through photosynthesis, appear to be at least as important as rain forests and other forms of terrestrial vegetation in controlling concentrations of the planet-warming gas, scientists said.

A study last year of yearly springtime "blooms" of phytoplankton in the North Atlantic Ocean revealed a huge, unsuspected reservoir of carbon, said Peter Brewer, a chemist at the Woods Hole Oceanographic Institution in Massachusetts.

Scientists now estimate that the phytoplankton, which exist in enormous numbers throughout the oceans, contain as much as 1.6 trillion tons of carbon -- at least as much as all of the trees, grasses and other land plants, he said.

The finding, growing out of a five-nation study of the North Atlantic phytoplankton, may explain the whereabouts of about 1 billion tons of "missing carbon" that seems to disappear from the atmosphere every year, he said.

Scientists from the United States, Britain, Canada, Germany and the Netherlands reported on various aspects of the ocean bloom experiment Monday at a symposium at the National Academy of Sciences.

Carbon dioxide has been building up in the atmosphere since the beginning of the industrial age because of increased combustion of oil, coal and natural gas, scientists say. Because of its tendency to trap heat, the gas is considered primarily responsible for the "greenhouse effect."

Through photosynthesis, phytoplankton and all other green plants remove carbon dioxide from the air or water and release oxygen. The process stores carbon in vegetation.

However, experts studying the global cycles of the gas had assumed that a much larger amount of carbon was stored in land plants than in the ocean's vegetation.

"We discovered a huge reservoir of dissolved carbon in the ocean, both in living and dead biomass," said Dr. Brewer, chairman of a steering committee overseeing U.S. participation in the North Atlantic Bloom Experiment.

With the advent of satellite imagery, scientists recently detected huge springtime "blooms" in which the amount of phytoplankton in the North Atlantic soars dramatically, apparently responding to slightly warmer water and more daylight hours.

Researchers set out to measure the degree of "drawdown" that phytoplankton may be making in the planetary carbon dioxide budget during the blooms.

The idea that phytoplankton constitutes a huge storehouse of carbon also raises serious issues about global climate change, said Dr. Brewer.

"We do not understand the controls on the size of this reservoir of carbon," he said. How the plankton responds to changes in the overall planetary temperatures and carbon dioxide levels could be much more important than scientists had dreamed, he said.

However, scientists not involved in the experiment reacted with skepticism to the suggestion that the world's "missing carbon dioxide" could be stored in the phytoplankton.